Generally, a construction machine like a hydraulic excavator which is built of a vehicular lower structure, an upper revolving structure swingably mounted on the vehicular lower structure, and a working mechanism liftably mounted at the front of the upper revolving structure and a ground excavating operation is performed by lifting up and down the working mechanism.
The upper revolving structure is largely constituted by a revolving frame having the working mechanism mounted at a front side, a cab which is mounted in a left front side of the revolving frame and an operator gets on board, a counterweight which is provided at the rear portion of the revolving frame as a weight balance relative to the working mechanism, an engine which is mounted on the revolving frame at a position on the front side of the counterweight, a heat exchanger which is composed of a radiator and an oil cooler which are located on one side of the transverse direction of the engine, an air intake pipe which is connected to an intake port of the engine, and an air cleaner which is connected to an upstream end of the air intake pipe and remove dust from engine intake air with a filter element.
On the other hand, there have been in use the so-called mini size hydraulic excavators each equipped with an upper revolving structure which is appreciably reduced in size to permit swing motions even in a narrow limited space free of interference with surrounding objects. More specifically, an upper revolving structure on a mini size hydraulic excavator of this sort is formed in a circular shape in section within limits of vehicle width of the vehicular lower structure. Therefore, an upper revolving structure does not have an ample space for onboard equipments. That is to say, a lot of onboard equipments are necessarily confined to a narrow space.
In this connection, the above-mentioned air cleaner, i.e., one of onboard equipments usually requires periodical cleaning or replacement of a filter element which is fitted in a casing. However, in the case of a small size hydraulic excavator, it is difficult to secure a sufficient space around the air cleaner for mounting and dismantling a filter element.
Therefore, it has been the usual practice for mini size hydraulic excavators of the prior art to mount an air cleaner vertically and mount or dismantle filter elements in the vertical direction. Accordingly, a filter replacement hole which lets filter elements pass through is provided at the bottom plate of the revolving frame positioned in a mounting or dismantling direction of the filter element. A cover plate is attached to the bottom plate of the revolving frame by the use of a plural number of bolts to close the filter replacement hole when there is no need for mounting or dismantling filter elements (see, for example, Patent Literature 1: Japanese Patent Laid-Open No. H9-221789).
By the way, in the invention of Patent Literature 1 just mentioned, considerable time and efforts are required for loosening and tightening a plural number of bolts which fix the cover on the bottom plate when mounting or dismantling filter elements of the air cleaner in a vertical direction, giving rise to a problem of low efficiency.
In addition, a filter element can be dropped by mistake due to difficulty of handling at the time of extracting a filter element from a casing of the air cleaner through a narrow filter replacement hole in the bottom plate of the revolving frame.